Enhancing small cell performance with synchronised backhaul

Timing synchronisation is fundamental to wireless network performance. Every individual radio transmitter throughout the network, effectively every cell, needs to be tightly aligned with its neighbours.

This doesn’t just benefit seamless handover as we move between cells, but reduces inter-cell interference, significantly increasing the total network throughput and capacity.

Three major trends are increasing its importance today:

Densification of the network, adding many more shorter range and lower power small cells.

LTE-Advanced (sometimes called 4.5G) added features such as eICIC, ABS and CoMP which require phase synchronisation in addition to frequency sync used for earlier LTE releases.

TD-LTE, already popular in China and India, is becoming available for use elsewhere and is particularly intolerant of jitter and delay variation.

Different sources of synchronisation

Network architects usually specify at least two alternative synchronisation sources to ensure network resilience and high availability. These include:

Timing Source

Advantages

Drawbacks

GPS/GNS

GPS inherently provides accurate phase timing of 30 nanoseconds or better. The introduction of other satellite constellations (GLONASS, BeiDou and Galileo) will increase the usefulness and resilience of this system.

Weak signals can be much more difficult to discern at street level in urban canyons with poor views of the sky. Potentially prone to jamming and multi-path.

Network Listen

Slaving from nearby macrocells.

Could cause entire area outage if macrocell goes offline. Isn’t suitable for small cells in coverage holes.

Requires an end-to-end implementation. Susceptible to failure in any link throughout the chain. The small cell backhaul network can be independent of macro network with unreliable CoS and high PDV.

Timing and sync within the wireless backhaul mesh

Wireless backhaul also requires accurate timing and sync to operate efficiently. Each backhaul node can be affected by the same issues for resilience between timing sources as do small cells. No single source is good enough on its own and the system must cater for a wide range of potential outages and use cases.

CCS Metnet self-organising wireless backhaul inherently incorporates accurate timing and synchronisation as a fundamental requirement for its own operation. Each node automatically switches to the most suitable synchronisation source available. The same three choices are available as for small cells: GPS, network listen slaved from another backhaul node or from an external wireline source.

Metnet passes this synchronisation timing signal over dedicated radio registration channels rather than bearer traffic channels, avoiding delay variation. Mesh topology, where most Metnet nodes have more than one diverse route back to the network, further adds to the high resilience of the backhaul. Switching between alternative routes is instantaneous and should not result in synchronisation loss.

The Metnet system can operate within a range of scenarios with a variety of synchronisation options. Inputs can include local nodes locked to GPS, local nodes locked to neighbouring GPS via the proprietary distributed GPS feature, or external synchronisation from the core networks i.e. frequency (SyncE) and phase and time (PTP 1588v2).

Outputs to the small cells include:

SyncE (G.8261/G.8262/G.8264) either generated directly from the Metnet GPS or from a core network device carried across the mesh.

1588v2 PTP T-TC (G.8273) from the core network boundary clock or grandmaster and propagated across the Metnet mesh.

Synchronisation as a service

Capitalising on the highly resilient and multi-sourced timing and synchronisation capabilities built into Metnet, CCS has developed proprietary mechanisms to transmit timing and sync for small cells.

Standard interfaces are provided for each small cell. This greatly simplifies the timing architecture by providing a robust and reliable timing source, connected through the same Ethernet cable used for backhaul and power. Each small cell receives a reliable SyncE and 1588v2 signal and does not have to deal with multiple sync sources or prioritise between them. Metnet seamlessly determines the best source and provides it in the same format.

While any timing source is available anywhere in the Metnet (GPS received at any node or from external SyncE/PTP source), then all small cells can remain synchronised.

Metnet flexible synchronisation options

Implications for small cell deployment

Identifying and arranging use of suitable sites for urban small cells is complex and difficult. Technical issues include ensuring the site is close enough to traffic hotspots, ensuring a power source, site security and access, and suitable backhaul. Commercial issues involve not just the site rental but planning/zoning permits, maintenance access (e.g. without closing off the street) and other costs.

Removing the technical constraints to have a good view of the sky for GPS reception increases the number of potential small cell sites that could be used. Mesh backhaul further relaxes selection criteria by catering for temporary line-of-sight disruptions and longer term ad-hoc network evolution.

A typical scenario is that one Metnet node with a clear view of the sky can be used as the primary timing source for the entire mesh backhaul and small cell network, including those located indoors or without adequate GPS reception.

Over the top synchronisation also supported

Some network architects may prefer to use their own centralised master clock sources. The robust timing architecture of Metnet also supports seamless carriage of SyncE and 1588v2 with low packet delay variation and high resilience meeting the G.8262 and G.8273 standards.

Best of both worlds

CCS has designed a high performance, robust and resilient timing and synchronisation solution as an inherent part of the Metnet mesh design. This capability can be used to provide a very accurate and low jitter timing signal to adjacent small cells.

Alternatively, for those who prefer to retain separation of backhaul and timing functions, these services can be passed through transparently.

This capability adds considerable flexibility to the scope of potential urban small cell sites, removing the need of a clear sky view for GPS satellite reception.